This invention relates to a digital filter device for tone control used in an electronic musical instrument or a tone generation device and, more particularly, to such digital filter device capable of realizing various filter characteristics as desired with a limited hardware structure.
An example of a filter having various characteristics employed in a tone color circuit of an electronic musical instrument is disclosed in Japanese Laid-open Patent Publication No. 55-45042. In this filter, plural voltage-controlled type filters having output terminals of different filter characteristics such as a high-pass filter output and a low-pass filter output are provided in parallel and the combination of individual filter characteristics is changed properly by changing the mode of connection of the respective voltage-controlled type filters such as by connecting a selected output terminal of one voltage-controlled type filter to an input terminal of another voltage-controlled type filter, and plural combined filter characteristics can thereby be selectively provided,
However, the above-described prior art filter, in which plural voltage-controlled type filters having output terminals of different filter characteristics must be provided in parallel as a hardware structure, is costly and requires a construction of large size. Besides, in the prior art filter, it is not possible to establish a filter characteristic independently for each channel when a construction in which tone signals are generated on a time shared basis in plural channels is employed, and, accordingly, this prior art filter is not suited to a device in which a filter control is made tone by tone by key scaling or the like technique.